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Evolving bacteria evade vaccines

Around 826,000 children die each year worldwide from infections caused by Streptococcus pneumoniae, and vaccination programs are cutting these numbers each year, as well as reducing the number of cases of infection passed on to other people. However, new strains of bacteria can evolve, and the vaccines become less effective, putting people at risk. A group of researchers from the U.K. and the U.S. have looked at how these changes can happen and what effects they may have. Details can be found in a paper published in Nature Genetics.

S pneumoniae bacteria have polysaccharides (sugars) on the surface, and the vaccines trigger an immune response against these sugars. The PCV7 pneumococcal vaccine, used in the U.S., is 7-valent--it targets seven of the 90 different polysaccharides (sugars) that are found in different strains of S pneumoniae.

After the PCV7 pneumococcal vaccine was introduced in the U.S. in 2000, a group of U.K. and U.S. researchers tracked the changes in S pneumoniae. They sequenced the DNA of samples of S pneumoniae and saw changes leading to new strains with different polysaccharides on the surface, which were not targeted by the vaccine. One new strain established itself quickly, spreading from east to west across the United States. The U.S. and the U.K. now use a vaccine that targets 13 of the polysaccharides.

Derrick Crook, professor of microbiology at the University of Oxford and infection control doctor at the Oxford University Hospitals NHS Trust, said: "Childhood vaccines are very effective at reducing disease and death at a stage in our lives when we are susceptible to serious infections. Understanding what makes a vaccine successful and what can cause it to fail is important. We should now be able to understand better what happens when a pneumococcal vaccine is introduced into a new population. Our work suggests that current strategies for developing new vaccines are largely effective but may not have long term effects that are as successful as hoped."

The results show how important it is to understand how vaccination can affect the strains of bacteria in the general population, and that genomic information can help researchers understand the evolution of disease, which will be important for the development of screening strategies and of new vaccines that could stay effective even when the bacteria change.